Here is the text of the proposed Tax Shelter and Tax Haven Reform Act of 2005 (S. 1565).  From the press release of co-sponsor Carl Levin (D-MI):

Abusive tax shelters are very different from legitimate tax shelters, such as deducting the interest paid on your home mortgage or Congressionally approved tax deductions for building affordable housing. Abusive tax shelters are complicated transactions promoted to provide large tax benefits unintended by the tax code. Abusive tax shelters are marked by one characteristic: there is no real economic or business rationale other than tax avoidance. As Judge Learned Hand wrote in Gregory v. Helvering, they are "entered upon for no other motive but to escape taxation."